This invention relates to an X-ray tube device with a rotatable anode, more particularly to a device whereby a rotatable anode target can be rotated at high speed while being supported in a non-contacting manner by magnetic bearings.
In an X-ray tube device with a rotatable anode, the target consists of a disk made of a refractory metal such as tungsten, and the X-rays are generated by making an electron beam collide with this target, whilst the target is being rotated at high speed. Rotation of the target is achieved by driving a rotor provided on a support shaft extending from the target. The support shaft is rotatably supported by means of bearings. Mechanical contact bearings have been used for this purpose. However they are liable to failure. This is because: (a) they have to support a heavy target which is rotating at high speed (at least 10,000 rpm); (b) they get very hot due to the heat from the target; and (c) they must provide support under vacuum.
Specifically, if the bearings are allowed to get hotter than 500.degree. C., the hardness of the bearing balls decreases. This may cause tube failure produced for example by stoppage of rotation. It has also been found that there is a severe reduction in bearing life (in terms of number of rotations) when the speed of rotation is increased, if the rotor and target are rotated under vacuum. In fact bearing life is unsatisfactory at the rotational speeds currently used in X-ray tubes (about 10,000 rpm).
Moreover, if the target weight is increased in an attempt to increase its heat capacity, this also leads to a reduction in bearing life. In order to overcome this drawback, magnetic floating X-ray tubes as described in U.S. patent specification No. 4,417,171 (Schmitmann), Japanese Patent Application Publication No. Sho. 58-43860, and Japanese Patent Application Laid-open No. Sho. 59-63646 were proposed. However, these are subject to the following drawbacks. In the case of U.S. patent specification No. 4,417,171, the external diameter of the rotor becomes very large, and in addition, since the supporting pillar at the centre must be at high voltage, it is difficult to hold. In the case of Japanese Patent Application Publication No. Sho. 58-43860, the target is of low rigidity and therefore has a low resonant frequency and cannot be rotated at high speeds. In the case of Japanese Patent Application Laid-open No. Sho. 59-63646, there is the inconvenience that not only must the anode be maintained at earth potential, but also a special high voltage power source and high voltage cable are required.